1. Technical Field
The disclosure relates to a tire air pressure monitoring device that monitors an air pressure of a tire of a vehicle and that issues a warning when the air pressure of the tire falls.
2. Description of Related Art
There is a tire air pressure monitoring device that issues a warning that a tire air pressure has fallen (referred to hereinafter as “a tire air pressure fall warning” or simply as “a warning” in some cases) when the detected tire air pressure becomes lower than a warning issuance threshold. This tire air pressure monitoring device stops (cancels) the warning if the tire air pressure becomes higher than “a warning cancellation threshold that is larger than the warning issuance threshold” when the warning has been issued.
To describe more concretely, the tire air pressure rises as a tire temperature (the temperature of air in a tire) rises. The tire temperature rises due to the running of a vehicle. Thus, as shown in FIG. 12, the tire air pressure monitoring device sets both the warning issuance threshold (see a solid line L5) and the warning cancellation threshold (see an alternate long and short dash line L6) in such a manner as to increase as the tire temperature rises (e.g., see Japanese Patent Application Publication No. 2010-143406 (JP 2010-143406 A)).
By the way, in general, an operator makes an adjustment such that the tire air pressure coincides with “a pressure Pr that is recommended when the tire temperature is equal to an outside air temperature”. This pressure Pr is referred to as “a placard pressure” or “a recommended set pressure”. Thus, the tire air pressure monitoring device issues a warning if the tire air pressure becomes equal to or lower than “a predetermined ratio of the placard pressure Pr (=a·Pr, 0.7≦a≦0.8)” when the tire temperature is equal to a standard outside air temperature Tr (see a point Q5 in FIG. 12). Accordingly, the tire air pressure monitoring device sets a value that passes through the point Q5 and that increases as the tire temperature rises, as the warning issuance threshold.